The spread of Klebsiella pneumoniae carbapenemases: a tale of strains, plasmids, and transposons.

Carbapenemases are a large and diverse family of microbial enzymes that hydrolyze not only carbapenems but also other b-lactams, with the occasional exception of monobactams, such as aztreonam. For clinicians, the nomenclature of carbapenemases, like that of other b-lactamases, is confusing. In keeping with other b-lactamases, carbapenemases can be classified on the basis of function or structure. The most commonly used classification—Ambler—is based on molecular structure. In this classification, carbapenemases may belong to classes A, B, or D [1]. Class A and D are serine carbapenemases, meaning that they have a serine at their active sites, like extended spectrum b-lactamases (ESBLs). In contrast, the class B enzymes are known as metallo-b-lactamases, because they require zinc as a cofactor [1]. Although the metalloenzymes were first described in the early 1990s, mostly in Pseudomonas aeruginosa, they remain uncommon in the United States [1]. A more recent arrival seems to pose a greater